High shear strength clay liner, method and apparatus for its production

ABSTRACT

A high shear strength clay liner comprising two synthetic geotextiles (1a, 1b) and a semihydrated layer (1c) of swellable clay therebetween with fastening means (5) arranged in such a pattern of connecting threads to form pockets (7) through the clay layer. The pattern comprises rows of independent fasteners arranged according two intersecting directions. The fasteners have knots (8) at their ends protruding from the surface of the liner. The invention also comprises a method and a machine for forming the independent fasteners by means of two parallel needle bars (104a, b) each carrying a row of needles (105) reciprocating in opposed directions and cooperating with a reciprocating needle plate (109) having a plurality of parallel fingers (110) and a looper bar (117) with a plurality of parallel looper plates (118) rocking on its axis to form loops (5a, 5b) which can be melted into protruding knots (8). The independent fasteners carry the major part of the shear forces on the liner avoiding failure of the fabrics and giving a much greater interfacial shear strength with a high internal friction angle.

FIELD OF THE INVENTION

The present invention relates to an improved clay liner to be used toform an impermeable barrier. In particular this invention relates to animprovement in the stability of the liner while being installed and alsoin the hydrated state while in use in steep slopes.

The present invention also relates to a method and an apparatus for theproduction of the clay liner of the invention.

BACKGROUND ART

Clay liners are widely used in the fields of Civil, Environmental andGeotechnical Engineering for waterproofing surfaces of soil, lagoons,foundation surfaces, and in particular constructions for the containmentof pollutant or toxic waste. The known clay liners are generally formedby a layer of swellable clay between two fabric layers (geotextiles).The clay used in this type of liner is predominantly Sodium Bentonite,wnich, when it comes into contact with water or other liquids, willswell to several times its dry volume if not restrained by a confiningstress. The swelling of the clay forms an impermeable barrier. The clayis usually confined between two fabrics and the clay and fabrics areheld together by either bonding with water soluble adhesive ormechanical bonding.

In adhesive bonding (see U.S. Pat. No. 4,501,788) the two fabrics andthe clay are held together by a glue which permeates through the clayholding the particles together and also holding the fabrics to eitherside of the clay layer. With mechanical bonding the clay is in dry formeither powder or granular and is held loosely in position betweenfabrics by needlepunching the two fibres together (see EP-A-278419).Needlepunching is a form of bonding using barbed needles which drag thefine fibre threads from one fabric to the other through the layer ofclay thus holding together the two fabrics with the loose clay between.The fibres dragged across are tangled with the fibres in the main bodyof the textile and not fastened in any way. The process is accomplishedon a machine that uses two needle boards one on the top and one on thebottom. These boards contain hundreds of barbed needles and move up anddown very quickly driving the rest of needles through the liner as itpasses between.

Like all clays when it is hydrated, which it must be to maintain itsimpermeability, it becomes very slippery and therefore unstable on asteep slope. To overcome the instability, due to a shear strengthfailure through the clay layer, the fabrics on either side of the claylayer must be connected to one another in some way through the claylayer.

The fine fibres mentioned in the needlepunched material also increasethe shear strength of the clay liner although this is not their primaryfunction. Needlepunched material has a number of disadvantages. Thestrength of the fine fibres connecting the two fabrics is low unless theintensity of the needlepunching is increased considerably, and if thisis done the increased needlepunching destroys the impermeability of theliner. A further problem with the needlepunching process is that anonwoven fabric must be used on at least one side of the liner to obtainthe fine fibres for the bonding. The nonwoven material allows a lateralflow of the liquid to be contained througn the plan of this materialgiving a problem with leakage at the edge, or overlap, of the liner. Ifa woven material is used, which is preferable in this type of liner, theweave would be damaged or destroyed by the barbed needles used in theprocess of needlepunching and weaken the liner.

In using this type of needlepunched liner on steep slopes there is alsoa problem with the movement of bentonite clay within the liner. The clayin its dry form, either powder or granules, is free to move and in theprocess of handling during loading, unloading and installation, tends tomigrate causing light areas with little or no clay and potential leakagein the depleted area. This can also happen when the clay is in a stateof hydration. When the clay is hydrated without cover. material to formthe confining stress, the hydrated bentonite becomes very swollen andsoft, if then the cover material is placed on the liner it will displacethe hydrated clay causing an increase in permeability and the liner toleak.

Another problem encountered with needlepunched clay liners is that theycannot be used in the field with a simple overlap. The seams must besealed with a dry bentonite clay powder or a bentonite paste which isplaced in position by hand. This is a very labour intensive operation,time consuming and costly. It does not always seal the seam and dependson proper supervision and human error. If two nonwoven textiles are usedthis particular problem is increased. It is very difficult for thelabour force to apply the seam sealing bentonite on steep slopes.

A sewn liner which is the only other mechanically bonded clay liner inuse, is disclosed in EP0490529. It consists of a glue bonded materialwith lines of stitching through the material in the machine direction togive a nigher friction angle. The stitches are lock stitches requiringone thread on too through the needle which then passes the threadthrough the liner, while a second thread on the bottom of the liner,supplied by a separate spool, locks the top thread. The machinery to dothis sewing is very expensive and the sewing can only be done in astraight line as it is impractical to form a pattern or deviate from thestraight line (too slow) and cannot be carried out with this machinery.Furthermore, this type of liner is subject to some problems manifest inthe needleounched liner. The glue bonded liner which is then sewn tocope with a steep slope situation, is initially bonded with a watersoluble glue to bond the fabrics of the liner and also the clayparticles together. The glue used in this process when dry becomes verybrittle and the process of sewing the two fabrics together tends tobreak up the dried glue and cause the bentonite granules to becomeloose. The bentonite is then free to move within the confines of thelinear sewing of the liner. This sewing is only in the machine directionand runs parallel the full length of the liner, the lines of sewingbeing some 100 mm apart. The loose bentonite material can migrate aconsiderable distance in the machine direction again causing areas oflimited bentonite and a potential failure of the liner. The solubleadhesive, being brittle when dry, can also cause this problem during thehandling of the sewn clay liner, when the liner is being loaded,unloaded and during installation on steep slopes where the dry loosebentonite will move down the slome in the machine direction between thelines of stitching which run in the same machine direction vertically upand down she slope.

The parallel stitching in the machine direction at 100 mm apart is theonly practical way in which this type of mechanical bonding can becarried out. Although the distance between the rows of stitches can bevaried, the stitches themselves can only run in a straight line in thedirection of production; this means that this problem is actuallywithout a satisfactory solution.

The same problem applies to the sewn liner when the bentonite ishydrated, as with the needlepunched liner, if any machinery orpedestrian traffic or cover material is placed on the liner. In itshydrated form the bentonite will be free to move in the liner within thelines of stitching causing an area of potential leakage where thebentonite is displaced. A lesser problem is the weight of the cover soilor other material which, pulling down the slope, will tend to increasethe tensile forces on the cross weave of the top fabric of the clayliner which is the weaker weave of the liner. This is due to thestitching being in line with the forces down the slope and not beingstaggered or offset to distribute the load or force acting in themachine direction.

EP-0611850 discloses a further type of geosynthetic clay liner includinga layer of bentonite disposed between a carrier sheet and a cover sheetwhich are linked to one another via a plurality of tufting threadsformed by a conventional tufting machine. A bight or loop portion of thetufting thread extends through to the undersurface of the carrier sheetand is partially melted to prevent the bight portion from slipping backthrough the carrier sheet. Due to the large number of tufting threadscrossing the bentonite layer there is no need for gluing the bentoniteto the inner side of the carrier sheet and the cover sheet. Thepartially melted bight portions create an undersurface with a highcoefficient of friction thereby improving the liner stability on steepslopes. However, the liner according to this disclosure is provided withfastenings arranged on parallel or staggered lines only and has an openpattern of the fastenings which causes the bentonite to be displacedunder heavy stresses or by gravity when the clay is either dry orhydrated.

Furthermore, in this liner the presence of a plurality of tufts (i.e.about 15 tufts or loops per inch as usual in tufting machines) not onlyincrease the permeability of the liner, but also weakens the fabric,thereby the liner is unable to withstand multidirectional high shearstress. The production process of this type of geosynthetic clay lineris also complicated by the fact that the liner must be sewn incorrespondence to its edges before being passed through the tuftingmachine.

Bentonite liners are used on the steep slopes of landfills to protectthe environment by forming an impermeable barrier on the slope. Thesteep slopes enable the landfill to be used more efficiently so that thelandfill will have a larger containment volume. This is also true ofother installations such as tank farms which have steep slopes on thesecondary containment areas in case of the tanks rupturing. Thebentonite liners therefore need to have higher shear strength, and thebentonite to be so confined that the forces acting down the slope willnot cause the liner to fail mechanically or the bentonite to becomeunstable when used on these slopes.

It is an object of the present invention to provide a flexible clayliner to form a impermeable barrier having a higher stability of theclay layer and higher shear strength with respect to the prior art clayliners, as well as stability on steep slopes and in standing water orinclement weather conditions.

It is another object of the present invention to provide a method formechanically bonding a geosynthetic clay liner of the above mentionedtype which allows the formation of independent fastening means forconnecting the fabrics containing the clay liner arranged according to aclosed pattern to confine the clay particles.

It is a further object of the present invention to provide a method formechanically bonding a geosynthetic clay liner of the above mentionedtype providing a liner with a surface coarseness adjustable on one orboth liner sides according to the needs.

Still another object of the present invention is to provide a machinefor mechanically bonding a geosynthetic clay liner of the abovementioned type operating according to the method of the invention.

The above objects are accomplished with the clay liner according to theinvention consisting of two woven geotextiles both of high tensilestrength in the machine and cross directions, a semihydrated layer orpowdered or granular swellable clay between said geotextiles andfastening means connecting the two geotextiles and arranged in such apattern of connecting threads to form pockets through the clay liner.The fastening means comprises rows of independent fasteners arrangedaccording two intersecting directions, forming an angle with thelongitudinal axis of the liner. Each fastener comprises at least a pairof sections of thread passing through the clay liner and the geotextilesand protruding therefrom to form a small loop on at least the undersideof the liner said loop being fixed and sealed to the geotextile surfaceor, preferably, melted into it to form a knot at each fastening site.

The method of the invention comprises the steps of:

preparing a geosynthetic clay liner, comprising two woven geotextilesboth of high tensile strength in the machine and cross directions and asemihydrated layer of powdered or granular swellable clay between saidgeotextiles, according to known methods;

forming on the geosynthetic clay liner symmetrically alternatedlongitudinal seams having a substantially zigzag path, each seam meetingthe adjacent one at least in correspondence of their edges or crests andbeing formed by threads passing through the liner in correspondance ofeach fastening site to form a closed pattern.

forming a protruding loop of thread at each fastening site on at leastone side of the liner;

locking the so formed loops to prevent them from unthreading.

As a preferred embodiment of the method of the invention, loops areformed on both sides of the liner and are then melted into thegeotextiles to form protruding knots on the independent fasteners.

The machine For introducing a geosynthetic clay liner according to theinvention and operating according to the above described methodcomprises a fixed machine frame, means for moving the liner along afeeding direction, a pair of parallel needle bars each carrying at leasta row of needles and arranged crosswise with respect to the feedingdirection and axially reciprocating in opposite direction with respectto each other, said needle bars further being reciprocated in adirection perpendicular to the plane along which the liner is movedthrough the machine so as to form corresponding seams extending in thefeeding direction with a substantially zigzag path. The machine furthercomprises a looper bar parallel to the pair of needle bars and placed onthe opposite side to them with respect to the liner, from the looper barthere extending radially looper plates in a side-by-side spacedrelationship, the looper bar having a rocking movement to engage eachlooper plate with the thread when the needle has gone to the end of itsdownstroke thus forming the loop. Means are provided for locking theloop and sealing the passage hole for the thread through the geotextileand means for moving in synchronism said needle bars and said looperarm.

In a preferred embodiment of the machine the means for locking the loopsand sealing the corresponding passage holes comprises singeing means formelting the loops and forming protruding knots therefrom. In a furtherpreferred embodiment a needle plate with a plurality of side-by-sidespaced apart fingers is provided in a parallel position with respect tothe needle bars and from the same side of them with respect to theliner. The needle plate is reciorocated axially in synchronism with theneedle bars so that each finger is alternatively placed at one side andat the other side of a corresponding needle while the latter is at theend of its upstroke. In this way at each fastening site loops can alsobe formed on the other side of the liner.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other features and advantages of the clay liner accordingto the invention will become apparent from the following detaileddescription, given as a non limiting example, taken in conjunction withthe attached drawings in which:

FIG. 1 is a perspective view of a slope to which the liner according tothe invention has been applied;

FIG. 2 is a cross sectional view of a first embodiment of the lineraccording to the invention wherein some fastener loops have been melted;

FIG. 3 shows diagrammatically the closed pattern of the threads on theupper fabric of the liner of the invention;

FIG. 4 shows schematically how the fasteners of the liner of FIG. 2 aremade;

FIG. 5 shows the spread of the tensile forces across the pattern of thefastenings in a clay liner according to the invention;

FIG. 6 is a cross sectionaly view of another embodiment of the lineraccording to the invention showing loops formed on both sides of theliner and singeing units to form individual fasteners with knots on bothsides of the liner;

FIG. 7 is a schematic side elevational view of the machine for producingthe liner of the invention;

FIG. 8 is a front view of the machine of FIG. 7;

FIG. 9 is a schematic perspective view of the machine of FIG. 7;

FIG. 10 is a partial side elevational view of the mechanical arrangementfor driving the needle bars in the machine of FIG. 7;

FIG. 11 is a top sectional view of the arrangement of FIG. 10 takenalong line XI--XI of FIG. 10;

FIG. 12 is a side sectional view taken along line XII--XII of FIG. 10;

FIG. 13 is a side sectional view taken along line XIII--XIII of FIG. 10;

FIG. 14 is a side sectional view taken along line XIV--XIV of FIG. 10,but with needle bars at their top dead centre;

FIG. 15 is a side sectional view taken along line XV--XV of FIG. 13, butwith needle bars at their top dead centre.

BEST MODE OF CARRYING OUT THE INVENTION

With reference to FIG. 1, there is shown a geosynthetic clay liner 1according to the invention on a prepared subgrade 2 of a slope of thetype used in both landfill and tank farm applications. The liner isusually 4 metres wide by approximately 25 metres long, however this canbe varied to requirement. The liner when in use would be locked into ananchor trench, not shown, at the top of the slope in the same way as anyother liner used in this field. The sealing at the seams or edges of theliners would be by a simple overlap. The overlap is usually about 15 cmwith the adjacent liner.

The liner is comprised of an upper geotextile 1a, a lower geotextile 1band a layer 1c of water swellable. clay. Geotextiles 1a and 1b arepreferably made of a woven, non-biodegradable polypropylene both of hightensile strength in the machine and cross direction. The water swellableclay is powder or, preferably, granular sodium bentonite, as usual.

Preferably the swellable clay is sprayed and/or mixed with water orliquid polymers to form a semihydrated clay mix with a moisture contentin the range of 15-20%. With the clay layer in the semihydrated form theliner is better processed in the fastening machine, as described later,because this machine as any other, is subject to considerable vibrationduring operation and this would substantially displace any loose orpowdered clay and cause variation in weight per unit area. Furthermore,the semi-hydrated mix is easier to keep in position than dry materialprior to fastening and dusty conditions are avoided during manufacturingand installation. The liquid polymer mix contains other usual chemicalsand additives to enhance the physical properties of the clay and to givethe liner 1 its high degree of flexbility. This enables the liner 1 tobe fitted to complex shapes and areas without the granules becomingloose. A typical chemical that can be added in this way is atrisodiumpyrophosphate compound or similar products to protect the clayfrom bivalent cations. A polymer mix that can be used with advantage isdisclosed in the International Application no. PCT/IT94/00041 in thename of the same Applicant.

The upper and lower geotextiles 1a and 1b are connected by fasteningmeans which in the present embodiment of the invention are formed bymutually intersecting rows 4 of fasteners 5 arranged according twodirections forming equal angles with the longitudinal axis of the liner,thus forming a diamond shaped pattern as shown in FIG. 1. The fasteningmeans can be arranged in various other closed patterns and the rows offasteners can be at various distances to give a variation in the size ofpattern. Other possible patterns could be square, zigzag or curvedpatterns, or many other shapes that might be preferred, to spread thedownward shear forces 20 on the clay liner 1 as shown in FIG. 5. In thecase of sewn liners, such as stitched clay liners, on the contrary, theshear forces are concentrated along the stitching lines down the slopethus increasing the risk of breakage of the thread.

According to one possible embodiment of the invention shown in FIGS. 2and 3, each row 4 of fasteners 5 is formed by aligned lengths of threadsa extending on the upper geotextile 1a according to intersectingdirections and passing through the clay layer 1c to form small loops 5aextending from the lower geotextile 1b. Each fastener 5 is thereforeformed by the two portions 5c and 5d of thread 6 passing through theclay layer 1c connected by a length of thread 6 to the adjacentfasteners on the upper geotextile 1a and forming the loop 5a on thelower geotextile 1b. The loops can be sprayed with a plastic andflexible coating, such as PVC, polyuretane or equivalent, to secure themto the geotextile or, as a particularly advantageous alternative, tomake each fastener 5 independent form the adjacent ones, the loops 5acan be melted into the geotextiles to form corresponding melted knots 8on the lower geotextile 1b thus preventing any unthreading of fasteners5.

The clay layer 1c is thus mechanically confined into independent pockets7 of clay, shown with dashed lines in FIG. 1, formed by portions ofintersecting rows 4 of fasteners 5. This substantially increases thefriction angle through the clay layer 1c and increases the tensilestrength of the whole clay liner 1.

In another, Presently preferred alternative shown in FIG. 6, loops 5aand 5b are Wormed on both sides of the liner, on the upper and lowergeotextile 1a and 1b, the loops 5b being formed by the lengths ofthreads 6. Each fastener is then constituted by a pair or short sectionsof thread with their ends being melted into the fabric on each surfaceof the liner forming a plurality of fastener heads or knots 8 evenlydistributed on both surfaces of the liner. The number of fasteners perunit length can be varied by varying the number of needles; aparticularly advantageous number is three fasteners per in (2,5 cm.approx.).

The thread is a yarn of similar tensile strength to the geotextile of apolymer, single or multifilament, that will not transmit liquid of anykind. For this purpose it can also be used with advantage a threadmanufactured by a technique known in the art as "fibrilation", thisprocess resulting in a net or grid configuration where no one fibremaking up the thread is continuous, which limits wicking of liquidsalong the thread. The material from which fasteners 5 are made can bechanged to any suitable material such as nylon, polypropilene,fiberglass, or other form of material to suit the economics or therequirements of the project under construction.

A clay liner according to the invention with a mechanical bondingbetween the upper and lower fabric forming a pattern of closed pocketsto confine the clay particles can be produced in the following way.

After forming a semihydrated clay layer 1c between two geotextiles 1aand 1b according to known techniques (for instance, see PCT/IT94/00041),a plurality of threads are passed through the so formed liner so as toform a substantially zigzag path extending lengthwise on the liner, inwhich two adjacent threads meet one another in correspondance to theangles of the zigzag path thus forming a closed diamond pattern. Thefasteners 5 are placed in position by a needle 10 and at least a looper11, as shown in FIG. 4. Needle 10 holding the thread goes down throughthe geotextiles 1a and 1b and the clay layer 1c and, as it rises fromits lower position, the thread is caught by looper 11 below the lowergeotextile 1b. When needle 10 moves upwards out of the liner, looper 11holds the thread to form loos 5a. As needle 10 comes down for the secondtime, the looper 11 moves backwards and the thread loop is released. Atthe same time, the liner moves forward, helping to clear the loop fromlooper 11. The needle 10 descends to its lowest position, a new loop isnow made and the cycle starts again.

In the case melting of the loops 5a is desired, the clay liner can bepassed over a heater 13 which will cause the loop thread to melt intogeotextile 1b fornming the independent fasteners 5 between geotextiles1a and 1b. The heater can be either of the gas for electric type or aninfrared neater, or equivalent.

In the case of a liner with loops 5a and 5b on both sides a furtherlooper, not shown, is used for forming the loops on upper geotextile. Inthis case the loops on the upper geotextile are formed by the lengths ofthreads 6 which are passed over the loopers by the travelling needles.If the loops on both sides have to be melted into knots 8, two heatingunits 13 must be used.

The individual fasteners formed with the above described method form aclosed pattern on the liner thus locking the clay between the fabricsinto the pockets of this pattern so that the clay cannot escape from theliner whether in dry form or hydrated. These fasteners between the twofabrics also set up a high shear strength making the liner very stableon steep slopes with a high internal friction angle.

The protrusion of the loops 5a, 5b and of the melted loops on thesurfaces of the liner 1 gives these surfaces a very rough finish. Theseprotrusions or the rough finish will lock into the material above andbelow the liner 1 and give a high interface friction angle at thesurface contact further improving the slope stability on steep inclines.This would be with the subgrade 2 below the liner 1 or the cover soilabove, or the protrusions would lock into any textured fabric ormembrane in contact with the liner. The heads of the fasteners lock intothe soil (or fabric) above and below the liner. The force of the topsoil (or fabric) acts against the head of the fastener in the directionor the force down the slope leaving a passive area behind the head ofthe fastener, thus reducing any force on the top textile of the liner.The same thing happens on the bottom fastener head, only in the reversedirection. This means that the main shear forces are carried by the hightensile strength fasteners and any forces on the upper and lowertextiles of the liner are greatly reduced. This gives better performanceof the liner on steep slopes, and as the fasteners are individual, thismakes the liner more reliable. This further provides the requiredtensile strength to prevent slippage of the soil cover and prevent creepfailure or damage to the fabrics and various geosynthetic components.The coarseness of either surface can be increased or reduced as requiredby the customer or as recommended by the design Engineer to suit theproject design by adjusting the size of the loops on the top or bottomsurfaces of the liner to increase or decrease the length so the loop andthen the size of the melted knot. This is obtained by adjusting thedistance of loopers from the liner travelling plane.

The melting of the loops or ends of the fasteners therefore providesthree main functions. First of all it forms the independent andindividual fasteners (which are unconnected, as seen in FIG. 6) joiningthe two fabrics of the liner through the clay layer to give the highinternal shear strength. Secondly, it seals the ends of the fasteners topreserve the low permeability of the liner, and thirdly, it creates atextured or rough surface interface friction when installed on a steepslope. Another important function is that the forces acting on the linersystem are mainly taken up by the individual fasteners, rather thanbeing taken up by the textiles as in all other types of liners in use.

The geosynthetic clay liner 1 is manufactured on a continuous basis andcut to the required length. The finished product is rolled onto acentral core to produce a roll of aporoximately 45 cm diameter. Therolled up liner can be then rolled out down the steep slope of alandfill or containment area to obtain more efficient use of the groundarea in the application and protect the environment. This also appliesto the steep slopes of tank farm bund walls and for pond liners. It canalso be rolled out directly onto a flat roof area to prevent leakagewhen used in conjunction with other roofing materials. The liner withupper and lower loops or knots can be installed with either surfacesbeing uppermost. The liner therefore can be installed using machinery tounroll and place the liner or can be installed by hand labour either onsteep slopes or flat installations at reduced costs of installation.

With reference to FIGS. 7, 8 and 9, the machine of the inventioncomprises a bridge-like frame 100 through which the geosynthetic clayliner L is massed continuously along a feed direction F. The liner to beprocessed can be supplied from a roll out unit 101 at the inlet side ofthe machine and wound again in a roll up unit 102 at the outlet sidethereof after processing. The liner is fed into and out of the machineby inlet and outlet pin rolls 103a,b which keep the liner in the correctposition while passing through the machine.

Two parallel needle bars 104a,b are supported by frame 100 transversallyto the liner feed direction. Each needle bar 104a,b holds a row ofneedles 105 and moves from side to side in opposite directions to oneanother indicated as 3 in FIG. 9, as well as up and down. The sidewaysmovement of the needle bars 104a,b is brought about by respective cams106a,106b located at the ends of the machine, driven through a gear box,not shown, from a main drive shaft 108, and connected to the needle barsby reciprocating shafts 107a,b as described in more detail below. Theprofiles of cams 106a,106b operate reciprocating shafts 107a,b throughcam followers, not shown, and control the sideways movement. The machineframe 100 also supports a needle plate 109 extending parallel to needlecars 104a,b and at the same side over the running liner. A plurality ofparallel fingers 110 extends from one longitudinal edge of needle plate109 towards the rows of needles 105. Needle plate 109 is supported bythe machine frame by a number of brackets 111 extending from the upperpart of frame 100 and holding guide bearings 112 into which is slidablymounted a shaft 113 connected to the needle plate by blocks 114 fixed toneedle plate 109. The needle plate 109 is also operated by a cam (notshown) at one side of the machine in the same way as needle bars 104a,bso that needle plate 109 can perform an alternating sideways movementserving both needle bars. The needle plate cam is operated by a camdriver through a gear reducer and timing belt (all not shown) from mainshaft 108.

A lower needle plate 115 situated on the underside of the liner supportsthe liner during the process and has spaced fingers 116 to allow thepassage or needles 105.

At the underside of liner L in correspondance to needle plate 109 alooper bar 117 is provided which extend crosswise to the liner traveldirection. Looper bar 117 carries radially a plurality of spaced looperplates 118 in the form of substantially angled fingers extending belowneedle plate fingers 110 and lower needle plate fingers 116. Looper bar117 is operated by a crankshaft, not shown, driven from main drive shaft108 by a timing belt, not shown. The crankshaft operating looper bar 117causes looper plates 118 to oscillate to perform a rocking motion asshown in FIG. 9.

Downstream the outlet pin rollers 103 a singeing unit 119 of theconventional type (for example a PAREX unit) is provided and formed, inthe present embodiment of the invention, by two gas fired units locatedat both sides of the liner.

As shown in FIGS. 10 to 15, where for sake of clarity only a portion ofone needle bar (104a) is shown, reciprocating shaft 107a has a radialarm 120 at one end carrying a pair of supporting shafts 121 held toframe 100 of the machine by a number of spaced guider bearings 122 inwhich they are slidably mounted for reciprocating motion. Supportingshafts 121 extend from one end of the machine for a length substantiallylower than that of the corresponding needle bar 104a. Two rollerbrackets 123 are clamped to supporting shafts 121 in a spacedrelationship and each carries a pair of spaced guide rollers 124 forsliding on a vertical guide member 125 placed therebetween fixed toneedle bar 104a and vertically extending from it. Needle bar 104a isalso provided with a number of linear bearings 126 along its length,each pair of them slidably supporting a pair of short guide rods 127 atthe centre of which a bracket 128 is clammed for holding a mush rod 129transmitting an up-and-down movement to needle bar 104a supplied by maindrive shaft 108.

It is clear from the foregoing that reciprocating shafts 107a,b throughrespective radial arms 120 operate supporting shafts 121 causing them tomove horizontally across the direction of travel of liner L for smalldistances governed by the cam profile. This action is transmittedthrough guide rollers 124 to vertical guide members 125 which are fixedto needle bar 104a causing it to move the required distance, One needlebar is operated by a cam at one side of the machine and the other needlebar is operated by a similar cam at the other side.

The up-and-down movement is transmitted to needle bar 104a through pushrods 129, which are fixed to machine frame 100, and guide rods 127,which slide into bearings brackets 126 integral to needle bar 104a.Bearing brackets 126 move needles bar 104a up and down while at the sametime allowing it to move from side to side as dictated by cam 106athrough supporting shafts 121, guide rollers 124 and vertically movingguide members 125.

The second needle bar 104b is controlled in the same way by cam 106b,reciprocating shaft 107b and other members as needle bar 104a at theother side of the machine which has exactly the same structure as thatshown in FIGS. 10 to 15 and described above for needle bar 104a.

In operation, the fabric of the geosynthetic clay liner L masses overthe inlet pin rolls 103a which hold the liner in position and in tensionwhile the two outer fabrics are fastened together. Liner L passes underneedle bars 104a,b to have the thread which will form the individualfasteners passed through the liner by needles 105 to create a closedpattern of the thread connectors or fasteners. One needle bar forms onehalf of the pattern and the other needle bar forms the other half of thepattern due to the movement in opposite directions of the two needlebars.

At each passage of needles 105 loops are formed on the upper and lowersurface of liner L. Loops on the upper liner surface are formed bysliding needle plate 109 which moves in a sideways movement back andforth. As needle bars 104a,b reach the top dead centre, the needle platecam is timed so that the needle plate 109 moves fingers 110, laying nextto needles 105, to the other side of needles 105 so that, when needles105 make their downward stroke, they are at the other side of fingers110, which causes the thread to be looped over each finger 110. Whenneedle bars 104a,b move the needles again to top dead centre the needleplate cam causes the needle plate to move the fingers back to the otherside of the needles and so continually forms these loops in the thread.Needle plate 109, therefore, moves one pitch to the right and one pitchto the left to form the loops on any variation of the pattern. Whenneedle bars 104a,b are on the bottom dead centre looper plates 118 onthe underside of liner L moves forward and pick up the thread to formlower loops. Once formed upper-and lower loops are pulled from fingers110 and looper plates 118 by the motion of the liner moving forwardthrough the machine. The rocking motion of looper bar 117 also helpsrelease the lower loop of thread. The movements of all these parts aresynchronized with the movement of needle bars 104a,b and are alloperated by main drive shaft 108.

After liner L has passed under needle bars 104a,b and the loops havebeen formed, it passes through outlet pin rollers 103b and between twosingeing units 119 containing heating elements and being continuousacross the full width of the machine and the liner. The heat from theseunits is set to a required temperature for the type of liner beingproduced and can be adjusted to suit the temperature needed to melt andseal the yarn fasteners. In particular, the heat is set to melt theloops to the required amount and can be adjusted if the loops are to belonger or shorter (for increasing or reducing the surface friction). Thesingeing units are electrically connected to the control board of themachine so that the temperature is automatically adjusted to the speedof the liner through the machine. The singeing units will alsoautomatically shield the liner from overheating if the machine isstopped. Under normal operation the loops of thread on either side ofthe liner will melt as they pass between the singeing unit. In doing sothe loops are broken and melt into a ball on the surface of the linerforming individual and independent fastenings between the upper andlower fabrics of the liner that are tightened up by the heat and theends of fastenings become heat sealed on both surfaces of the liner.

In the present embodiment of the invention, the needles 105 in theneedle bars 104a,b are spaced at 2.5 inches (6,25 cm). The fingers 110on needle plate 109 are spaced at 1/8 inch (3 mm) across the full widthof the machine, the looper plates 118 on the under side of the liner arealso spaced at 1/8 inch (3 mm) across the full width of the machine. Thefingers of the sliding needle plate 109 and the loopers 118 are used bythe needles 105 in-needle bars 104a,b. Half of the fingers 109 andloopers 118 are used by needle bar 104a and half are used by needle bar104b. The large number of fingers 110 and loopers 118 available improvesthe flexibility of the operation of the machine by the ability to changethe pattern of the fasteners.

The pattern formed on the liner can be varied in size, type and shame bychanging the profiles of cams 106a,b which control the sideways movementof needle bars 104a,b. Though she machine according to the invention hasbeen described as fed by separate rolls of prepared liner, it is obviousthat it can be constructed as a part of a production line and fed with acontinuous liner.

From the above it can be seen by those experienced in the fields ofCivil, Environmental and Geotechnical Engineering that a liner of thistype has many engineering and economic advantages in terms of a)reliability, due to the fasteners taking the majority of the shearstresses away from the body and fabrics of the liner while maintaining ahigh internal friction angle through the clay layer; b) slope stability,due to interlocking of the fasteners into the upper and lower surfacesof the installation in which the liner is being used; c) performances ofthe liner as an impermeable barrier and due to the increase in strengththat can be provided by the variation in size, type, thickness and depthof the fasteners and the textiles used in the manufacture of the liner;and d) cost, adjustable to suit the project and due to the fact that inmany cases the use of this liner will dispense with other forms ofreinforcement that are necessary with other liners. It has also to benoted that, while in prior art geosynthetic clay liners such as thatdisclosed in EP-0611850, a plurality of stitches or tufts (15 per 2.5 cmapprox.) are required to obtain the required layer stability, with theconsequence of a substantial weakening of the liner and increase ofpermeability, in the liner according to the invention, due to the closedpattern and the pocket structure of the bonding, no more than 3fasteners are 2.5 cm approximately are requested. The use of bentoniteliner in vertical or steep slopes can be made with heavier covermaterial with the obvious advantages which cannot be achieved by similarliners. This liner can also be used without cover in these conditionsand will not be affected by heavy rain or inclement weather. If requiredthe cover material can be applied after hydration of the clay. The linercan be installed in standing water dispensing with the need for drainageif necessary. This enables its use in preventing roof leakage.

We claim:
 1. An impermeable liner comprising two synthetic geotextiles(1a, 1b) with a semihydrated layer (1c) of swellable clay between saidgeotextiles and fastening means (5) connecting the two geotextiles,wherein said fastening means (5) are arranged in such a pattern to formpockets (7) through the clay layer and are arranged in rows ofindependent unconnected fasteners (5) arranged in two intersecting rows(4) forming an angle with the longitudinal axis of the liner.
 2. Theliner according to claim 1, wherein each fastener (5) comprises at leasta pair of sections of thread (5c,5d) passing through said clay layer(1c) and said geotextiles (1a,1b) and protruding therefrom to form asmall loop (5a) on at least one side of the liner.
 3. The lineraccording to claim 2, wherein each fastener (5) further comprises asecond loop (5b) protruding from the other side thereof.
 4. The lineraccording to claim 3, wherein said loops are secured by a plasticflexible coating sprayed on them.
 5. The liner according to claim 1,wherein each fastener (5) comprises at least a pair of sections ofthread (5c,5d) passing through said clay layer (1c) and said geotextiles(1a,1b) and protruding therefrom with a small knot (8a) on the undersidethereof.
 6. The liner according to claim 5, wherein the knot comprises aball.
 7. The liner according to claim 1, wherein said geotextiles(1a,1b) are of woven high tensile, polypropilene.
 8. The liner accordingto claim 1, wherein said thread is a high tensile, single-ormulti-filament yarn.
 9. The liner according to claim 1, wherein saidloops or said knots protrude from the sides of the liner by a variableextent so that the surface coarseness can be adjusted to the needs foran improved frictional resistance.
 10. The liner according to claim 1,wherein said clay layer is sodium bentonite clay mixed with a liquidpolymer mix to form a semihydrated clay layer.
 11. The liner accordingto claim 1, wherein a number of fasteners not greater than three per 2.5cm approx is provided along each row of fasteners.
 12. The lineraccording to claim 1, wherein the connecting threads comprisefibrillated thread.
 13. A method of making a geosynthetic clay linercomprising:disposing a layer of semihydrated swellable clay between asynthetic geotextile first liner and a synthetic geotextile secondliner; disposing a thread through a series of pairs of holes locatedoppositely in the first liner and in the second liner, such that on thefirst side a plurality of first loops are formed between adjoining onesof the holes in the first liner, and on the second side a plurality ofsecond loops are formed at respective individual holes in the secondliner; and melting the first loops and the second loops; whereby each ofthe second loops forms a respective ball and each of the first loopsforms a pair of balls, the pair of balls being located at the adjoiningones of the first holes.
 14. A geosynthetic clay liner made by theprocess of claim
 13. 15. The method according to claim 13, wherein thestep of disposing the thread further comprises:stitching the first andsecond liners such that the series of pairs of holes located oppositelyin the first liner and in the second liner form a first zig-zag pattern;and stitching the first and second liners such that the series of pairsof holes located oppositely in the first liner and in the second linerform a second zig-zag pattern; and positioning the first zig-zag patternand the second zig-zag pattern to form pockets.
 16. A geosynthetic clayliner made by the process of claim
 15. 17. An impermeable liner,comprisingtwo synthetic geotextile layers with a semi-hydrated layer ofswellable clay between said geotextile layers, and a plurality ofthermoplastic threads extending through said geotextile layers andswellable clay therebetween, said thermoplastic threads connecting saidgeotextile layers together, said thermoplastic threads each having afirst end and a second end with enlarged portions of said thermoplasticthread at each of said first and second ends, said thermoplastic threadsextending throughout their respective lengths generally perpendicular tosaid geotextile layers, said enlarged ends of said first and second endsof said thermoplastic threads abutting respectively against exteriorsurfaces of said geotextile layers, said thermoplastic threads beingarranged in rows, a first plurality of said rows extending in onedirection and a second plurality of said rows extending in a seconddirection, said first plurality of rows intersecting said secondplurality of rows at an angle with the longitudinal axis of the liner.18. An impermeable liner comprising two synthetic geotextiles (1a, 1b)with a semihydrated layer (1c) of swellable clay between saidgeotextiles and fastening means (5) connecting the two geotextiles,characterized in that said fastening means (5) are arranged in such apattern to form pockets (7) through the clay layer and comprises rows ofindependent unconnected fasteners (5) arranged in two intersectingdirections (4) forming an angle with the longitudinal axis of the liner,wherein each fastener (5) comprises at least a pair of sections ofthread (5c, 5d) passing through said clay layer (1c) and saidgeotextiles (1a, 1b) and protruding therefrom with a small knot (8a) onat least the underside thereof, and further a protruding knot (8b) onthe other side thereof.
 19. The liner according to claim 18, wherein theknot comprises a ball.